Optical frequency conversion, such as second harmonic generation (SHG), third harmonic generation (THG), or sum or differential frequency generation, is employed in laser systems to generate coherent beams of light at a variety of wavelengths ranging from infrared to ultraviolet. A frequency-converted laser system has a laser oscillator coupled to a frequency converting nonlinear optical crystal. An amplifier may also be coupled between the oscillator and frequency converting crystal. The laser oscillator is often operated in a pulsed regime to attain the high peak intensity levels required for efficient nonlinear optical conversion. A monochromatic laser beam, called a “fundamental” beam, is focused into one or more nonlinear optical crystals, where the frequency conversion takes place, to generate a “harmonic” beam, for example a second harmonic or a third harmonic optical beam.
When nonlinear optical frequency conversion of a laser beam to the ultraviolet or other short wavelengths is performed in a crystal, any optical absorption can result in heat deposition within the crystal. This deposited heat can lead to an increase of the crystal temperature. The degree of temperature increase depends on the optical absorption level and amount of optical power transmitted or generated in the crystal. If large enough, the temperature rise in the crystal can cause changes in the optimal phase matching angle within the crystal and, in turn, cause changes to the beam position, beam pointing, or the conversion efficiency, depending on the exact configuration.
In particular, in the case of relatively tight focusing within the crystal to achieve high conversion efficiency, a crystal temperature change will cause a change in the angular pointing direction of the harmonic beam out of the crystal. This pointing change occurs in a phase-matching plane. Since the pointing direction depends on the amount of absorbed optical power, any changes in the input or output optical power can cause pointing changes. Thus, the beam pointing will change as the optical power levels are increased or decreased, which is very undesirable in an end application, such as laser machining. Similarly, if the optical power level is modulated on and off, the beam pointing direction will depend on the present modulation state and the recent thermal history of the nonlinear crystals.
Kuhl et al. in U.S. Pat. No. 3,962,576 disclose a frequency-converted laser system using SHG, in which nonlinear optical crystal temperature and/or orientation is adjusted to keep the SHG efficiency high. To that end, a pair of photoelectric detectors is used to determine a change of a relative position of fundamental and second harmonic optical beams. When a change of the relative position is detected, a feedback circuit causes the nonlinear optical crystal to be rotated, or its temperature changed, so as to counteract the detected change of the relative position of the beams. This improves stability of SHG efficiency.
Govorkov et al. in U.S. Pat. No. 6,614,584 disclose a system operating similarly to that of Kuhl et al. In the Govorkov system, separate position sensitive detectors (PSD) are used to track the locations of the fundamental and harmonic beams. The nonlinear optical crystal orientation and/or temperature are adjusted to keep the relative position of the beams constant, which results in keeping an optimal phase matching condition in the crystal.
Adjusting orientation of nonlinear optical crystals is associated with a fundamental drawback, namely it can cause a displacement of the frequency-converted laser beam. In many applications, laser beam positioning and pointing need to be stable with time. Whether a frequency-converted laser beam is coupled into an optical delivery fiber or is reflected towards a target in free space, an unstable or wandering laser beam can cause imperfect or fluctuating illumination of a target, or can even cause a catastrophic failure of a laser beam delivery system.
Wang in U.S. Pat. No. 7,242,700 discloses a frequency-converted laser system, in which a power and a position of a frequency-converted laser beam are monitored. The temperature of the nonlinear optical crystal is adjusted to maintain the frequency-converted beam at a pre-determined position. The optical pump power is adjusted to maintain the power of the frequency-converted beam at a predetermined level. Thus, the crystal temperature adjustment serves to stabilize the pointing, whereas the pump power is adjusted to stabilize the output optical power.
One drawback of stabilization of the output beam pointing by tuning the crystal temperature is a relatively slow response time. An oven or thermostat holding a SHG or a THG crystal is typically designed with a significant thermal mass to ensure that the crystal is held at a uniform temperature over its entire length. Changing the temperature of the oven and the crystal can take from tens of seconds to a minute. When the optical power is changed, one must wait this long before a thermal drift of beam pointing can be fully compensated. This reduces the utility of the laser system, particularly if rapid changes between power levels, or a gated operation regime is desired.